1. Field of the Invention
This invention relates to the detection of the fundamental frequency of an alternating current input wave and to audio synthesizer systems which utilize the fundamental frequencies of audio waves and, more particularly, to the detection of fundamental frequencies in real time, especially for realistically synthesizing audio frequencies.
2. Description of the Related Art
The detection of the basic or fundamental frequency of a wave, often called f0, is of importance in many fields including metrology, radio, television, radar, sonar, telephony, many types of scientific and medical instrumentation and electronic music. Determination of the single cycle integral of a wave in real time also has uses in many of these and other application areas.
Determination of the fundamental frequency (f0) of a wave which has been obscured by high harmonic content or noise has been solved for some cases by various means known to the art. Despite this, certain waveforms, including those generated by the human voice, have defied all attempts until now to determine f0 in real time. By real time is meant on a cycle by cycle basis, with the duration of each cycle becoming known at essentially the moment of its ending. None of the following related prior art solves the problem of detecting the fundamental frequency in real time.
Malka U.S. Pat. No. 4,644,268 issued Feb. 17, 1987 for Apparatus and Method for Determining the Magnitude and Phase of the Fundamental Component of a Complex Waveshape. Malka discloses a method for "Determining the Magnitude and Phase of the Fundamental Component of a Complex Waveshape" with respect to a reference wave. Malka assumes that the amplitude of the input wave changes slowly so that digital computer means, having sampled the wave, can adjust amplifier means to bring it into a usable measurement range. It is assumed it will stay there for subsequent measurement. The input wave is then digitized, with the first digitized sample timed from the zero crossing of the reference wave. Fast Fourier Analysis and other computer means are then used to calculate the magnitude of the input wave and its phase difference from the reference wave. Very useful for calculating the angular position of resolvers and other devices which shift the phase of a reference proportional to some physical variable.
Schoenberg U.S. Pat. No. 4,457,203 issued Jul. 3, 1984 for Sound Signal Automatic Detection and Display Method and System. Schoenberg discloses a method for determining frequency based upon the assumption that an input wave has only one positive and one negative peak per cycle. Automatic Gain Control and variable frequency lowpass filter means are applied to the incoming wave so that the cycle detection criteria may (hopefully) be satisfied. During AGC and filter cutoff point adjustment, cycles may be "lost". Subsequent computer means compensate for this loss by application of statistical methods to the series of measurement values to determine input frequency and display same.
Girgis U.S. Pat. No. 4,319,329 issued Mar. 7, 1982 for Frequency Measuring and Monitoring Apparatus, Methods and Systems. Girgis discloses a frequency measurement method based on the assumption of a single positive going axis crossing per cycle from a "cleaned up" filtered waveform. The cycle is digitized and the use of a variant of the Fast Fourier Transform method calculates precise and relatively small deviations from a known frequency, the known frequency having previously been used to calibrate the system.
Katterfeld U.S. Pat. No. 4,161,625 issued Jul. 17, 1979 for Method for Determining the Fundamental Frequency of a Voice Signal. Katterfeld discloses an enhancement of the autocorrelation technique. This technique is based upon the supposition that sequential cycles of a complex wave will vary little from one to the next. Thus, by taking a time "windowed" sample of the wave and continuously comparing it to the incoming wave, it may be expected that a "match", or very close, correlation between sample and incoming wave will be found one cycle later. The process requires continuous sampling and comparing.
Some of the approaches taken by the field of speech recognition are disclosed in Digital Processing of Speech Signals, by Rabiner & Schafer, Prentiss-Hall 1978. Disclosed are several techniques for finding fundamental frequency: estimations based on zero crossings, autocorrelation and short term average magnitude difference function (a variant on autocorrelation, see Katterfeld above). The authors conclude (at p. 135): "All of the proposed schemes have their limitations and it is safe to say that no presently available pitch detection scheme can be expected to give perfectly satisfactory results across a wide range of speakers, applications and operating environments."
Some of the approaches taken in the field of electronic music are disclosed in Musical Applications of Microprocessors, by Hal Chamberlin, Second Edition, Hayden Book Company, 1985. A survey of pitch detector schemes is disclosed including all those in the Rabiner & Schafer book and the above referenced patents. A series of "troublesome waveforms for pitch detectors" is presented, one or more of which will cause every scheme to fail. The conclusion (at p. 578) is that there is an "abundance of pitch detection schemes covering a wide range of complexity and performance levels. Even so, it is safe to say that none of them is completely satisfactory, that is, agree with a human observer in all cases."